System for adjusting the wavelength light output of a semiconductor device using hydrogenation

ABSTRACT

A method and structure for adjusting the wavelength output of a semiconductor device is described. In the method, the hydrogen concentration in an active region of the semiconductor device is adjusted either during fabrication or after the device has been fabricated. The adjustment provides a simple technique for fine tuning many device types including regular lasers and VCSEL structures. The adjustment also allows for mass production of lasers of many different frequencies on a single wafer substrate, a system particularly desirable for wavelength division multiplexing systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is related to U.S. patent application Ser. No.11/300,861 entitled “High Power Semiconductor Device with Low-AbsorptiveFacet Window”, and U.S. patent application Ser. No. 11/304,221 entitled“Buried Lateral Index Guided Lasers and Lasers with Lateral CurrentBlocking Lasers”, and U.S. patent application Ser. No. 11/300,871entitled “On-Chip Integration of Passive and Active Optical ComponentsEnabled by Hydrogenation”, all assigned to the same assignee and filedon the same day on Dec. 15, 2005, and all are hereby incorporated byreference.

BACKGROUND

The growing role of wavelength division multiplexing (WDM) in opticalcommunication systems has driven the need for semiconductor lasersources that can emit at different wavelengths. Current WDM systems forexample use over 100 wavelengths spanning a range from about 1525 nm toabout 1600 nm. With the advent of Raman amplifiers and new highbandwidth filters, WDM transmission wavelengths are expected toencompass the so called S-band nearing 1300 nm. Lasers based on InGaAsNcan potentially cover the entire wavelength range from 1200 to 1600 nm.

Fabricating a number of semiconductor lasers spanning a wavelength rangehas proven to be a difficult challenge. One method is to adjust thecomposition of the active material by changing the growth recipe of eachlaser device. In particular, the active layer of each laser may differaccording to the desired output frequency. The variation may be achievedby varying the epitaxial growth of each active layer as described inU.S. Pat. No. 6,167,074 entitled “Monolithic Independently AddressableRed/IR Side by Side Laser” by Sun et al. filed Feb. 24, 2000 and herebyincorporated by reference.

One problem with a multiple recipe approach is the difficulty ofimplementation in manufacturing settings. In particular, applyingdifferent recipes can result in delays and reproducibility problemsassociated with recipe changes. Multiple recipes also increase costs.For example, different reaction chambers may typically be used toaccommodate different recipes. Some implementation would involve aregrowth. Etching and regrowth processes are undesirable because of thehigh cost associated with pre-regrowth sample preparation, the epitaxialregrowth process itself, and the manufacturing logistics involved. Anadditional drawback is the non-planar morphology that results.

Alternative approaches to produce laser arrays with varying emissionwavelength are based on growth techniques such as migration-enhancedepitaxy and temperature-graded substrate condition. However thesetechniques are complicated, time consuming and are difficult toprecisely control. Failure to maintain tight controls results indifficulty controlling the emission wavelength.

Thus a simpler and less expensive way of tuning the wavelength output ofan InGaAsN laser is needed.

SUMMARY

A method for tuning a semiconductor device to output light is described.The method involves forming a semiconductor structure including anactive region. The amount of hydrogen in the active region is changed toreach a desired wavelength. The technique may be used for VCSELs or fortuning to different frequencies lasers in an array of lasers. The tuningmay be done during or after the laser is fabricated. In one embodiment,hydrogenation may be used to shape the lateral index changes. Theselateral index changes may be used to control the polarization of thelaser output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph plotting the output photoluminescence spectra ofInGaAsN/GaAs quantum wells doped with various quantities of hydrogen.

FIG. 2 shows a plot of the absorption characteristics of a semiconductoras a function of the incident photon energy.

FIG. 3 shows a cross sectional view of a laser structure that relies onhydrogenated facets to minimize absorption.

FIG. 4 shows a mask with apertures being used to control hydrogenationof a wafer.

FIG. 5 shows a buried index guided laser diode structure usinghydrogenated InGaAsN and GaAsN layers for lateral index guiding.

FIG. 6 is a table showing different confinement factors and effectiverefractive indexes for various example structures.

FIG. 7 is a plot of lateral confinement factors for a buried lateralindex guided laser structure versus waveguide width for differentrefractive index steps.

FIG. 8 shows an example of a ridge-waveguide laser diode.

FIG. 9 is a schematic that shows different contributions to the netoptical gain of a laser as a function of wavelength.

FIG. 10 shows an array of lasers coupled to gratings, each laser outputsa different frequency of light, all lasers may be on the same wafer.

FIG. 11 shows a wafer in a vacuum chamber as one method of selectivelyhydrogenating regions of a wafer.

FIG. 12 shows a half tone mask using different aperture densities tocontrol hydrogenation of a wafer.

FIG. 13 shows a VCSEL where the hydrogen content of the active regionmay be adjusted to tune the frequency of the laser output.

FIG. 14 shows a cross sectional view of an index guided opticalwaveguide where a half tone mask is used to create a desired lateralindex variation.

FIG. 15A is a side cross sectional view of the integration of an opticalwaveguide with a laser diode. A half tone mask is used in this case togradually modify the lateral and vertical index profile. FIG. 15B showsthe index profile at two sample locations in the structure of FIG. 15A.

FIG. 16 shows a three dimensional view of a various regions of asemiconductor waveguide coupling a glass fiber to a semiconductor laser.

FIG. 17 is a cross section view of a two section laser diode structurewith a modulator and amplifier region.

FIGS. 18-21 show various integrated optical circuits that may befabricated on a single semiconductor wafer using the describedwaveguides.

FIG. 22 shows a plot of an asymmetric index profile plotted along alateral or vertical dimension and how the index profile affects theoptical mode.

FIG. 23 shows a plot of a symmetric index profile that may be used, forexample, in a beam splitter and how the index profile affects theoptical mode.

DETAILED DESCRIPTION

A system for forming and interconnecting optoelectronic devices isdescribed. The system controls exposes a semiconductor compoundincluding at least GaAsN to hydrogen in a hydrogen concentrationadjustment process. More typically, the GaAsN is an alloy that includesat least one of either aluminum to form AlGaAsN or Indium to formInGaAsN. For convenience, the specification will refer to InGaAsN,although it should be understood that the invention should apply to anysemiconductor material that changes bandgap as a result of exposure tohydrogen, and the most common semiconductor materials that exhibit thisproperty are alloys, typically AlGaAsN or InGaAsN. By controlling theamount of hydrogen in the sample, the bandgap of the sample can becontrolled thereby controlling the sample's refractive index as well asabsorption properties.

As used herein, “altering” or “changing the hydrogen concentration” isbroadly defined to include all methods of increasing or decreasing theconcentration of hydrogen in a material. The adjustments may beaccomplished by either removing previously incorporated hydrogen or byadding hydrogen atoms and/or molecules. Incorporation or otherwiseadding hydrogen to a material may be accomplished using varioustechniques, including but not limited to, diffusion through exposure ofa sample to a hydrogen gas. In one example, a layer is grown by MetalOrganic Vapor Phase Epitaxy (MOCVD), where hydride precursor gases suchas ammonia are typically employed. Moreover, hydrogen is usually thecarrier gas of choice for flowing metal organics for building theepitaxial structure. The hydrogen-rich environment in the entire growthprocess makes it easy to incorporate hydrogen into the material duringepitaxial growth. Masks may be used to control the regions that areexposed to hydrogen.

Alternately, changing hydrogen concentrations may refer to a hydrogenion implantation wherein high energy hydrogen particles are directed atthe sample to selectively incorporate hydrogen into a sample. Ionimplantation, selective areas of the material can be masked off byphotoresist during device fabrication. The unmasked surfaces can then beexposed to standard hydrogen ion implantation accelerated at, typically,between 50 KeV to 350 KeV depending on the penetration depth desired. Inpractice, the exact dosage and energy levels required is determinedexperimentally using Secondary Ion Mass Spectroscopy (SIMS)characterization in conjunction with electroluminescence measurements.Monte Carlo computer simulation techniques using software such as thepopular SRIM and TRIM packages can also be employed to model the ionimplantation process. Sources of hydrogen include, but are not limitedto low energy Kaufman sources and plasma hydrogenation systems.

As defined and used herein, altering or changing hydrogen concentrationsis not limited to increasing hydrogen concentrations. Hydrogenconcentration changes also include processes of removing previouslyincorporated hydrogen atoms from a pre-hydrogenated sample. An examplemethod of reducing hydrogen concentration is to anneal portions of asample, typically at temperatures about 500 degrees centigrade or aboveduring fabrication. Controlled removal from specific regions may beeffected by patterning a region with a “hydrogen getter” such aspalladium and then heating the sample. Electron beam irradiation orwavelength selected laser writing also provide methods of localizedheating for selective area hydrogen removal.

Changing the hydrogen concentrations in InGaAsN materials produces abandgap shift in InGaAsN materials or samples. The process is describedin G. Baldassarri, M. Bissiri, A. Polimeni, and M. Capizzi,“Hydrogen-induced Band gap Tuning of (InGa)(AsN)/GaAs Single QuantumWells” Appl. Phys. Lett., Vol. 78(22), pp. 3472-3474, (2001) which ishereby incorporated by reference. The effects of hydrogen on the InGaAsNbandgap may be measured by observing the photon energy of thephotoluminescence peak of InGaAsN/GaAs quantum well at differenthydrogen concentrations.

FIG. 1 (taken from the above cited Baldassarri reference) plots theoutput photoluminescence spectrum of a plurality ofIn_(0.34)Ga_(0.66)As_(1-y)N_(y)/GaAs quantum well structures as afunction of quantum well hydrogen concentrations. The dashed curves 104,108, 112 show the photoluminescence of a reference sample with y=0 (thusno nitrogen) while the solid curves show the photoluminescence of areference sample with y=0.007. H_(O) in the figure represents a hydrogenconcentration that results from exposures to a hydrogen flux of 1×10¹⁶hydrogen atoms/cm²

In the absence of hydrogenation, the energy difference between thephotons output by an InGaAsN active region and the photons output by anInGaAs (no nitrogen) active region is approximately 0.05 electron volts(eV). In particular, curve 116 peaks around 1.01 eV indicating themajority of photons output by InGaAs has an energy of 1.01 eV. Curve 104peaks around 1.06 eV indicating the majority of photons output byInGaAsN have an energy of around 1.06 eV. Increasing hydrogenationlevels (increased hydrogen concentrations) increases the output photonenergies of nitrogen containing materials. In particular, at aconcentration of 690 H_(O) (a concentration resulting from a flux of690×10¹⁶ hydrogen atoms/cm²) shown in curve 120, the bandgap ofIn_(0.34)Ga_(0.66)As_(0.993)N_(0.007)/GaAs shifts by about 60 meVresulting in a bandgap similar to that of a nitrogen free referencesample shown by curve 112.

Changing the bandgap of a material not only alters the light output by amaterial, it also has other effects. For example, changing the bandgapchanges the refractive index of a material and also the photonabsorption properties of the material. As the InGaAsN material bandgapincreases due to increased hydrogen concentration, the refractive indexdecreases. A typical change in the index of refraction is an index shiftfrom about 3.68 to an index of about 3.52 for light with a 1.3micrometer wavelength. The associated refractive index shift isdescribed in T. Kitatani, M. Kondow, K. Hinoda, Y. Yazawa and M. Okai,“Characterization of the Refractive Index of Strained GaInNAs Layers bySpectroscopic Ellipsomtery,” Jpn. J. Appl. Phys. Vol. 37, pp. 753-757,1998 which is hereby incorporated by reference in its entirety. Bycontrolling the refractive index change in different parts of a device,optical devices and interconnects may be formed as will be describedbelow.

FIG. 2 plots a material's absorption properties as a function ofincident photon energy. The absorption is plotted on axis 204 and theenergy of an incident photon plotted on axis 208. Photons below abandgap energy 212 are not absorbed and thus the thus the materialappears transparent to photons below bandgap energy 212. Photons withenergies above the bandgap encounter some absorption. Curve 216 showsthat as the photon energy increases, the absorption coefficient alsoincreases. Changing the hydrogen concentration shifts bandgap energy 212along axis 208 resulting in changes to the material absorptionproperties. As will be described, this property will be used to lowerthe absorption of the facets in a laser diode.

A number of applications can be made using the previously describedproperties. In one example, the lateral as well as vertical hydrogenconcentrations are manipulated to modify the bandgap and refractiveindex. Step changes in the index of refraction can be used to formwaveguide boundaries. However, graded or gradual index variations mayalso be used to form waveguides or control optical mode profiles.

One example of using index changes to control modal profile are shown inFIGS. 22 and 23. FIGS. 22 and 23 show a plot of a varied index profile2204 and 2304 plotted along a lateral (or vertical dimension) (axis d).The index profiles affect the optical modes of light propagating throughthe varied index profile. Curves 2208 and 2308 show example resultingoptical modes for each index variation curve. More details on the use ofboth the stepped index variation and the graded index variation will bedescribed in the devices that follow.

High Power Semiconductor Light Output Devices with Low-absorbtive FacetWindows

The described methods may be used to alter the facets of semiconductorlight output devices, although particular use may be found in atraditional high powered long wavelength InGaAsN system laser. As usedherein, “semiconductor light output device” is broadly defined toinclude any device that uses a gain region of a semiconductor materialto amplify or to generate light. Example semiconductor devices includelasers, amplifiers. As used herein, “facets” are broadly defined to meanthe region immediately adjacent an active region through which laserlight passes before exiting the laser. The facet is often made ofmaterial very similar to the active region. The “active region” isdefined as the area of a semiconductor device that generates and/oramplifies light usually by stimulated or spontaneous emission of light.Thus “active region” is typically the gain medium of a laser, or anamplifier. In a LED (including superluminescent LEDs) the “activeregion” is usually the region in which spontaneous emission of lightoccurs. As previously described in the background, laser facets oftenabsorb some of the laser's output energy. The absorbed energy heats thefacet causing defects and bandgap shrinkage. The resulting defects andbandgap shrinkage increases facet energy absorption sometimes resultingin catastrophic optical damage and laser failure.

In order to avoid the catastrophic damage, one exemplary embodiment ofthe invention adjusts the concentration of hydrogen in the facet region.In particular, hydrogen content in the InGaAsN and/or GaAsN facet regionis increased to increase the bandgap in the facet regions rendering thefacet less-absorbing of photons output by a semiconductor device. Inparticular, facet hydrogenation makes the facet bandgap slightly largerthen the bandgap of the active region.

Various methods may be used to change the hydrogen concentration in thefacet. One method of increasing the hydrogen concentration is hydrogenimplantation in the facet region. A second method is simply to allowhydrogen to diffuse into the cleaved facet region of the semiconductorlaser. FIG. 3 shows a laser 300 exposed to hydrogen 304 such that thehydrogen diffuses into one or both laser facets 308, 312. Such hydrogenincorporation may be achieved by cleaving the laser to create laser endsor laser facets. The cleaved facets, typically laser facets 308 and/or312 are then exposed to monatomic hydrogen at about 300 degreescentigrade. The rate of diffusion depends on temperature and can bemodeled theoretically as described, for example, in Physics ofSemiconductor Devices, 2^(nd) Edition, S. M. Sze, John Wiley & Sons,1981, pp. 66-69 and in “Modeling of hydrogen diffusion in p-typeGaAs:Zn”, R. Rahbi, et. al., Physica B: Physics of Condensed Matter,Volume 170, Issue 1-4, p. 135-140. The diffusion behavior can bedetermined experimentally using standard Secondary Ion Mass Spectroscopy(SIMS) characterization techniques. A typical diffusion depth of betweenabout 500 nm to 5 μm is desirable. A typical exposure may occur for aperiod of time of between 1 and 15,000 minutes in a chamber withhydrogen at a pressure of 10 to 800 torr. Facet HR coating orpassivation may occur after exposure and diffusion of the hydrogen intothe facet. The hydrogen levels are typically increased until a bandgapshift of between 1 meV and 100 meV is achieved.

Sometimes, processing of individual lasers is slow and a wafer levelprocessing is needed. FIG. 4 shows one example of masking a wafer 400.In wafer level processing, the hydrogenation of the facets may becompleted prior to cleaving or sawing the wafer to create individuallasers. In the embodiment shown in FIG. 4, a metal or dielectric mask408 covers wafer 400. The mask 408 includes opening 412 positioned overwhere the laser will eventually be cleaved. The wafer 400 is thenexposed to hydrogen atoms 416. Hydrogen atoms impinging the wafer 400through openings 412 and diffuse into the active layers. In an alternateexemplary embodiment, ion implantation drives hydrogen into the facetarea prior to laser facet cleaving. In still a third exemplaryembodiment, the entire wafer is hydrogenated and hydrogen then hydrogenremoved from non-facet areas. Removal may be accomplished by selectiveheating of non-facet areas or by masking non-facet areas with a hydrogenattracting material. After the hydrogen is embedded into the laserfacets, the wafer may be cut or cleaved to form individual devices suchas lasers, LEDs and the like.

In regions that include nitrogen, hydrogen increases bandgap in facets308, 312 such that the facet bandgap is larger than the bandgap inregion 316 between the facets. Thus, InGaAsN layer 320 includes a facetregion 324 that has a slightly larger bandgap than active region 328.Likewise, GaAsN spacing layers 332 includes facet regions affected byhydrogenation and thus have a bandgap larger than regions that do notcontain nitrogen. Examples of regions that do not contain nitrogeninclude AlGaAs guiding layers 336 and AlGaAs:Si cladding layers 340. Atypical example hydrogen concentration in the laser facet 308, 312region exceeds 5×10¹⁶ hydrogen atoms/cm² which results in a bandgapshift that exceeds 20 meV. In one embodiment InGaAsN active regionoutputs light at 1.48 micrometer which results from a bandgap of about0.84 eV. Very few photons of this energy are absorbed by the laser facetwith a bandgap that typically exceeds 0.86 eV.

Besides increasing the bandgap, hydrogen in the facets also reduces thefacet index of refraction. A reduced waveguide refractive indexdifferential between the guiding layers 336 and the facet 308, 312increases beam spreading at the facet. Beam spreading lowers the beampower density in the facet which also helps to reduce facet heating. Inone embodiment, the lateral index of refraction in the facet is gradedto provide additional control of the optical modes. The optical modesmay be molded to further enhance beam spreading of light from the activeregion of the semiconductor device. Alternately, the optical mode may beshaped for input into a receiving device or waveguide.

High powered long wavelength high reliability lasers such as 1.48micrometer lasers are particularly useful for optical communicationsystems. The 1.48 micrometer wavelength which can be generated by anInGaAsN laser, is particularly suitable for pumping erbium-doped fiberamplifiers often used in long haul optical communication systems. Such acommunication system is described in Understanding Fiber Optics, 2^(nd)Edition, Jeff Hecht, Sams Publishing, 1993, ISBN 0-672-30350-7,pp.112-113 which is hereby incorporated by reference.

Pump wavelength at 1.48 μm allows longer and more uniform fiberamplifier gain sections compared to the alternative 980 nm pumpwavelength because of lower fiber absorption at 1.48 μm. Strongabsorption of the pump source at 980 nm results in weak pumping of thesignal at greater distances from the laser source. Thus, InGaAsN pumplasers at 1.48 μm is desirable, and the use of hydrogenated facets inhigh power InGaAsN pump lasers allows these lasers to operate with longlifetimes due to minimized facet heating.

The signal wavelength in long haul communications systems is typicallyaround 1.55 μm. InGaAsN active layers can be designed for 1.55 μm laseroperation. 1.55 μm is the signal wavelength of choice in long haulfiber-optics communication systems because of the wavelength's lowabsorption in typical fibers. High power at 1.55 μm is advantageousbecause it allows the signal to travel a longer distance beforerequiring amplification thereby allowing fewer costly amplifiers in thecommunication link. Thus the high powered signal lasers that output 1.55μm also benefit significantly from reduced facet heating. Thus both thepump lasers and the signal lasers operate under high power conditions,the reliability of which can be enhanced by minimal absorption andheating at the laser facet. The increased reliability is particularlyvaluable in applications involving hard to access areas such as underseafiber links.

Burried Lateral Index Guided and Lasers with Lateral Current BlockingLayers

In order to improve the performance of semiconductor light outputdevices, usually semiconductor lasers, traverse index guiding mechanismsare used to laterally index guide the active region. In the followingdescription, a hydrogenation-induced bandgap shifted (HIBS) materialalong at least one edge of an InGaAsN active region serves as thelateral index guiding structure. Such a structure is shown in FIG. 5.

FIG. 5 shows an InGaAsN square quantum well active region 504 sandwichedbetween GaAs (N) spacers 508, 512 and AlGaAs cladding layers 516, 520,524, 528. As used herein “sandwich” merely means between two layers, andnot necessarily adjacent to either of the two layers. In the illustratedembodiment, closer cladding layers or guiding layers 516, 520 areundoped while lower cladding layer 524 is silicon doped and uppercladding is Carbon doped. Substrate 532 supports the structure whilecontacts 536, 540 allow electrical pumping of active region 504. Such anInGaAsN laser stack is described in many prior art references includingU.S. Pat. No. 6,922,426 entitled “Vertical Cavity Surface Emitting LaserIncluding Indium in the Active Region” by Johnson which is herebyincorporated by reference.

Hydrogenated lateral region 544, 548 bordering active region 504 guidesphotons generated by active region 504. As used herein, “lateral” isdefined as regions to the side, typically in a wafer that includesmultiple layers, lateral regions to an region in a layer, (such as anactive region), will be other regions in the same layer. Typically, thelateral region is made from the same material and layers as the materialin other regions of the InGaAsN laser except that the lateral regionshave been hydrogenated. In one embodiment, lateral regions are merelyhydrogenated regions of a laser layer. Thus, when an InGaAsN activeregion is created from an epitaxially grown layer, the lateral regionsare merely portions of the InGaAsN epitaxially grown layer that has beenhydrogenated. Adding hydrogen increases the lateral region bandgap anddecreases the index of refraction. Both characteristics enhance indexguiding of the wave propagating in active region 504.

Various methods may be used to change the hydrogen concentration inlateral regions 544, 548. In one example method, hydrogen ionimplantation is used to direct hydrogen to the target area, lateralregion 544, 548 in the illustrated example. In a second example method,monatomic hydrogen may be diffused through openings in a masked wafersurface. The masking structure of FIG. 4 may be used, except thatopenings 412 in the mask correspond to surrounding lateral regions 544,548 instead of laser facets. Thus, in this second method diffusionoccurs in a vertical direction. In still a third method, hydrogen may beallowed to laterally diffuse from an etched ridge into surroundinglateral regions 544, 548. In particular, a ridge may be etched adjacentto lateral regions 544, 548 and the ridge exposed to a hydrogen gas.Different temperatures and pressures may be used to incorporate thehydrogen, one example of which is described in hydrogen diffusion andacceptor passivation in p-type GaAs from R Rahbi et al. APL 73 pp.1723-1731 (1993). In the cited reference, hydrogen was diffused in thesamples by exposure to a hydrogen rf plasma ( 13.56 MHz) at a constantpressure of 1.2 mbar. The exposure temperature was chosen in the 50-300Centigrade range for different durations of 30 min to a few hours.

As described earlier, changing the hydrogen concentration is not limitedto adding hydrogen to a region. In still a fourth method ofhydrogenating the lateral regions, hydrogen may be introduced to theentire layer including active regions and lateral regions. Excessivehydrogen may then be removed, particularly from the active region, byselectively heating the active region or by placing the active region incontact with a material with a strong affinity for hydrogen.

FIG. 6 and FIG. 7 illustrate the resulting confinements that may beobtained using lateral hydrogenation. FIG. 6 shows confinement factors(ratio of the modal gain to the active region gain) and effectiverefractive indexes for example InGaAsN laser structures. Rows 604 and608 of FIG. 6 shows effective index changes in which only the InGaAsNactive layer lateral regions 552, 556 are hydrogenated. Rows 612 and 616show effective index changes when both the InGaAsN active layer lateralregion 552, 556 and the GaAsN spacer lateral region 560, 564 arehydrogenated resulting in an active region that is bounded on all fourssides by hydrogenated material. The sample computations are done for an8 nm wide InGAsN SQW active region and a 35 nm wide GaAs(N) spacerlayer.

In FIG. 6, column 620 provides confinement factors and column 624provides effective refractive indexes for a zero order mode for variousexemplary structures described in Column 628. Row 604 provides theconfinement factor and the effective index of refraction of the centralregion, including a GaAs spacer, all of which has not undergonehydrogenation. Row 608 provides the confinement factor and the effectiverefractive index of the adjacent surrounding lateral region when onlythe lateral portion of the InGaAsN active region is hydrogenated but thelateral portion of GaAs spacers have not been hydrogenated. Box 632shows the difference in effective refractive index produced byhydrogenating only the lateral portion of the InGaAsN active region. The0.004 effective refractive index difference provides some index guiding,however, a larger change in index would provide stronger index guiding.

An order of magnitude increase in the index differential may be achievedby not only hydrogenating the active region, but also hydrogenating theadjacent GaAsN spacer lateral region 560, 564. Row 612 provides theconfinement factor and the effective index of refraction of a centralregion that has not undergone hydrogenation. Row 616 provides theconfinement factor and the effective refractive index of the adjacentsurrounding lateral region when both the lateral portion of the InGaAsNactive region and the lateral portion of GaAsN spacers have beenhydrogenated. Box 640 shows that an approximately 0.042 difference ineffective indexes results between the effective index of anon-hydrogenated structure and the effective index of a structure whereboth the lateral active region and lateral spacers have beenhydrogenated.

FIG. 7 plots percentage lateral confinement along axis 704 as a functionof waveguide width along axis 708 for various example refractive indexdifferences. The example data is for a laser that outputs 1.3 micrometerwavelength light. FIG. 7 illustrates that an effective refractive indexof 0.04 shown in curve 712 provides significantly greater lateralconfinement at smaller waveguide widths than an effective refractiveindex of 0.004 shown by curve 716. The advantage is less pronounced whenwider waveguides are used, however wider active areas are typically notpreferred due to difficulties with carrier confinement.

Surrounding the active region with higher bandgap materials not onlyprovides optical index guiding, the higher bandgap can also improvelateral carrier confinement because the lateral increase in the bandgapenergy prevents or at least reduces lateral diffusion of the injectedcarriers. Carrier confinement becomes exponentially more effective withincreasing band offsets, so it is desirable to maximize the band offsetby incorporating as much hydrogen in the lateral cladding area aspractically possible. The physics of carrier confinement is describedin, for example, chapter 3 of Physics of Optoelectronic Devices, by ShunLien Chuang, John Wiley & Sons, 1995, ISBN 0-471-10939-8. Carrierblocking regions enable more efficient current injection resulting inhigher power conversion efficiency. Higher efficiencies enable narrowerwaveguide regions for single mode devices because fewer carriers arelost to lateral diffusion. In order to further enhance current blocking,nitrogen may be used not only in the active light-emitting layers, butalso in adjacent cladding and waveguide layers. Introducing nitrogeninto the cladding layers allows hydrogenation to bandgap shift a thickerlayer and thus further improving lateral carrier confinement.

Although FIG. 5 shows a buried index guided laser diode structure, itshould be understood that the described index guiding and currentconfining properties may also be used for other laser types. Forexample, the described hydrogenation techniques may be used inridge-waveguide laser diode shown in FIG. 8. In ridge-waveguide laserdiodes, hydrogenation can be done by simple diffusion from the etchedridge 804. The hydrogenation in this example provides an additional“soft” confinement of the optical mode which reduces e.g. lightscattering at the etched mesa edges. Alternate gradual control ofhydrogenation will be described in connection with waveguides in FIG. 14and the accompanying description, although the gradual hydrogenationcontrol may also be used to guide the signals in the active region of alaser.

Variable Wavelength Lasers and Laser Arrays.

As modern networks need to carry more information on optical fibers,Wavelength division multiplexing (WDM) emerged as an important method ofincreasing the carrying capacity of an optical fiber. In WDM, differentwavelengths of light are simultaneously transmitted on a common fiber.Each wavelength carries its own information. The different wavelengthsare separated or demultiplexed by devices at a receiving end of thefiber. A typical DMW system uses several semiconductor light outputsources, typically semiconductor laser sources that emit at differentwavelengths. One example WDM system uses an array of lasers thattogether are capable of outputting over 100 wavelengths spanning a rangefrom 1525 nm to about 1600 nm. Eventually, WDM systems are expected toencompass the S-band nearing 1300 nm, the wavelength approximatelyoutput by InGaAsN systems. However, fabricating the laser array tooutput the many different wavelength outputs is difficult and thusexpensive.

Laser wavelength output depends on several factors. Typically a lasertends to lase at the mode frequency with the greatest net gain (netgain=stimulated emission minus optical losses), see, Yariv, A. 1991Optical Electronics, 4^(th) edition (Holt, Rinehart and Winston). Thus,selecting an output wavelength involves adjusting the greatest net gainposition.

FIG. 9 shows the various elements that can go into determining the netgain. Net gain is typically determined by the sum of the material gainshown in curve 904 and a free spectral range curve shown in curve 908.Curve 904 shows the gain in the active material. A peak 906 of materialgain curve 904 typically corresponds to the bandgap of the activematerial where the average output energy of a photon is approximatelyequivalent to the bandgap. Curve 908 corresponds to the free spectralrange of the laser which is determined by the active material lasercavity dimensions. The laser output typically stabilizes at a frequencyat the spectral range peak 910 that is closest to active material peak906. Thus by adjusting the bandgap of the active material usinghydrogenation, the output of the laser may be selected.

One problem with the illustrated structure is that the peak gain of theactive material shifts with temperature. FIG. 10 shows an example arrayof semiconductor lasers including a semiconductor laser cavity 1004surrounding active material 1008. Furthermore, processing changes makeit difficult to repeatedly create a bandgap peak at a desired frequency.Thus a grating may be added to provide feedback to each laser in thelaser system. In FIG. 10, each semiconductor laser outputs light into acorresponding grating 1012. The grating provides a feedback signal thatdetermines the laser mode selected. This type of laser is called thedistributed Bragg reflector (DBR) laser. A discussion of how modeselection is determined relative to grating dimensions and materialparameters is given in, for example, pp. 95-101, Diode lasers andPhotonic Integrated Circuits, by Larry Coldren and Scott Corzine, JohnWiley & Sons, 1995, ISBN 0-471-11875-3. The feedback provided by grating1012 is graphically illustrated by curve 912 of FIG. 9 where the laseroutput wavelength is determined by grating feedback peak 916.

When fabricating an array of different lasers with different outputwavelengths for a WDM system, different gratings are used to produce thedifferent laser outputs. One fabrication challenge is to approximatelymatch active material curve peak 906 to the grating feedback peak 916.Peak mismatches result in inefficiencies, and in the worst case,insufficient total gain to maintain continuous stimulated emissionoutput. Thus, the active material bandgap should be adjusted therebyadjusting active material curve peak 906 to approximately match gratingfeedback peak 916.

One method of adjusting active material curve peak 906 is to change thegrowth recipe of each laser device. In particular, the active layer ofeach laser may differ according to the desired output frequency. Thevariation may be achieved by varying the epitaxial growth of each activelayer as described in U.S. Pat. No. 4,839,899 entitled “WavelengthTuning of Multiple Quantum Well (MQW) Heterostructure Lasers” by Burnhamet al. which is hereby incorporated by reference. However, this multiplerecipe approach is difficult to implement in manufacturing settings. Inparticular, applying different recipes can result in delays andreproducibility problems associated with recipe changes. As used herein,recipe means any sequence of instructions and parameters used to form aparticular device. Multiple recipes also increase costs due to theincreased length of time required to build the device.

Instead of altering each active region during growth, one exemplaryembodiment of the invention adjusts the wavelength output of each laserby hydrogenation-induced bandgap tuning (hereinafter HIBS). HIBS adjuststhe bandgap of each InGaAsN laser by changing the amount of hydrogen ineach laser's active region. Use of HIBS allows multiple lasers designedto output different wavelengths to be grown using the same reactor andthe same recipe and even on the same common InGaAsN layer. After growth,the hydrogen content in each active region of each laser is adjusted. Aspreviously described and illustrated in FIG. 1, increasing the hydrogenconcentration increases the bandgap of the InGaAsN/GaAs quantum wellsthereby shortening the wavelength at which active material peak 906 willoccur. Thus shorter wavelength lasers with higher hydrogenconcentrations in the active material will correspond to lasers thatoutput shorter wavelength light and are thus coupled to gratings withshorter periodicities or spacings between adjacent grates.

The same hydrogenation and grating shift techniques used DBR lasers mayalso be used in Distributed Feedback Lasers (DFB). In a DFB laser, thegratings are formed above the active layer instead of beyond it as inDBR lasers. DFB lasers are described in, for example, pp. 102-108, Diodelasers and Photonic Integrated Circuits, by Larry Coldren and ScottCorzine, John Wiley & Sons, 1995, ISBN 0-471-11875-3 which is herebyincorporated by reference.

Using the above described methods, an InGaAsN laser may typically betuned in a range from 1.3 to 1.55 micrometers. The hydrogenconcentration determines the output frequency which determines thegrating spacing used. For example, if an InGaAsN laser is designed tooutput a wavelength of 1.55 μm, a typical grating pitch separation ofabout 502 nm (half wavelength pitch with 114.7 nm of epi and 387.5 nmair gap) is appropriate. The actual grating separation can be scaledaccording to the material refractive index to produce half wavelengthpitch separation if materials other than air are used between gratingfeatures. Higher order grating geometries where the pitch is made aninteger number of half wavelengths is also possible. Shorter wavelengthemitters on the same substrate have grating pitches accordingly scaledshorter to maintain a half wavelength pitch.

In one embodiment, the active material is hydrogenated to shift the peakmaterial gain closer to the target wavelength. FIG. 1 approximates theamount of hydrogen needed for a desired shift. Increasing hydrogenconcentrations increases the bandgap towards that of a nitrogen-freealloy. Bandgap adjustments of up to 50 meV may be obtained atsufficiently high hydrogen dosage for In_(0.34)Ga_(0.66)As_(1-y)N_(y)with a nitrogen content y of 0.007. Larger nitrogen concentrationsenable even larger bandgap shifts by hydrogenation.

Various methods may be used to change the hydrogen content in the activeregion of a semiconductor laser. One previously described technique usesion implantation. Another method that is particularly suitable tofabricating multiple semiconductor lasers on a wafer that outputdifferent wavelengths is to form a mask over the wafer. FIG. 11 shows aset up that may be used to hydrogenate laser active regions of a wafer.

In FIG. 11, a wafer 1104 is placed in a vacuum chamber 1108. Byproviding different level of hydrogenation various devices can befabricated thereon based on a single common epitaxial layer, such assemiconductor lasers, amplifiers, detectors, waveguides and similarstructures. A vacuum pump 1112 removes reactive and other unnecessarygasses along flow path 1116. A hydrogen source 1120 provides hydrogen.Hydrogen source may typically obtain the hydrogen by many methods,including electrolysis of water or decomposition of ammonia. Also H₂ orplasma hydrogenation could be used. Hydrogen is provided along flow path1124 into vacuum chamber 1108. The wafer is heated to a temperaturebetween 150 and 500 degrees centigrade, and more typically between 250and 400 degrees centigrade to facilitate diffusion of hydrogen into thewafer. The exposure period varies according to the level ofhydrogenation desired.

A mask deposited over wafer 1104 may include apertures that correspondto the active regions of each laser. In one embodiment, a half tone maskwith different density of apertures or openings over each active regionmay be used. Areas with a higher density of apertures will result inhigher levels of hydrogen incorporation and undergo a larger bandgapshift. FIG. 12 shows an example mask that includes aperture regions 1204and 1208. The higher density of openings in aperture region 1204 resultsin a higher hydrogen density and thus a larger bandgap. Thus the laserthat will be associated with aperture region 1204 will output a shorterwavelength. In alternate embodiments, the apertures may be differentsizes with larger apertures being used to simulate a higher density ofapertures. In still a third alternative embodiment, instead ofapertures, the mask may be made of a material partially permeable tohydrogen such that the rate of hydrogen flow is controlled by thethickness of the mask.

Post Growth VCSEL Tuning

The described method of tuning a semiconductor light output device,typically an active region of a laser is not limited to edge emittinglasers or even lasers. The tuning technique described may also be usedfor the tuning of the active region of almost any semiconductor devicethat outputs light including, but not limited to light emitting diodes(LEDs including superluminescent LEDs), amplifiers and vertical cavitysurface emitting lasers (hereinafter VCSEL). An example VCSEL 1300 shownin FIG. 13. The VCSEL tuning may occur during or after VCSEL devicefabrication.

A VCSEL typically includes an active region, here an InGaAsN activecavity 1304. A bottom set of distributed brag reflectors (DBR mirrors)1308 supports a bottom surface of active cavity 1304. In the illustratedembodiment, bottom set of DBR mirror stack 1308 includes 35 pairs of DBRmirrors typically formed from AlGaAs. An upper set of DBR mirror stack1312 bounds a top surface of active cavity 1304. In the illustratedembodiment, upper DBR mirror stack 1312 include approximately 25 DBRmirror pairs formed from AlGaAs.

The DBR mirror stacks are similar to gratings in that the stack feedsback a particular frequency of laser light equivalent to grating curve912 of FIG. 9. The vertical dimensions of active cavity 1304 determinethe free spectral range or internal cavity curve 908 of FIG. 9. In oneembodiment, it is desired that the thickness of the active cavity 1304be approximately three times the wavelength of the desired VCSEL lightoutput.

Low cost coarse WDM systems typically have wavelength spacings of about25 nm between channels, while dense WDM systems use wavelength spacingsof less than 4 nm. However during actual fabrication it is difficult toform the cavity structure with such precision. Thus, wavelengthdifferences from run to run and wafer to wafer are unavoidable due tolayer thickness and material composition variations that occur duringepitaxial growth. Emission wavelength differences of up to many tens ofnanometers are not unusual, even among devices on the same wafer.

In order to compensate for those wavelength differences, the activematerial in VCSEL active cavity 1304 may be exposed to hydrogen (orhydrogen removed) to compensate for the variations which occur duringprocessing. In particular, out of specification VCSELs may have hydrogenconcentrations adjusted (either increased or decreased) to “tune” theVCSEL to output the desired frequency.

The mechanism of how a VCSEL wavelength is changed using hydrogenationwas described by FIG. 9 and the accompanying description. The VCSELlaser cavity 1304 may be viewed as a structure similar to active lasercavity 1008 except that the wave propagation in VCSEL 1304 is in avertical direction. Thus, similar to tuning laser cavity 1008, theoutput wavelength of the VCSEL may be tuned by shifting curve 908 ofFIG. 9 such that the peak occurs at or near the desired frequency. Thus,if the wavelength output by a VCSEL is too long, the output wavelengthmay be reduced by increasing the hydrogen concentration in theInGaASN/GaAS quantum well. If the output wavelength is too short,hydrogen may be removed from the active region. As previous described,increased hydrogen concentration increases the active material bandgapresulting in a shorter wavelength output.

Various methods may be used to adjust the hydrogen concentration in theVCSEL. In one embodiment tuning is done by exposing the VCSEL tohydrogen. In particular, the VCSEL may be exposed to a hydrogen gas at atemperature of 300 degrees centigrade to facilitate incorporation ofhydrogen. Alternately, hydrogen ion implantation may be used to embedhydrogen atoms into the material. In still an alternate embodiment,hydrogen may be incorporated into the VCSEL structure during epitaxialgrowth, and the adjustment may consist of annealing out excess hydrogento tune the VCSEL after fabrication.

The VCSEL material is grown by Metal Organic Vapor Phase Epitaxy(MOCVD), where hydride precursor gases such as ammonia are typicallyemployed. Moreover, hydrogen is usually the carrier gas of choice forflowing metal organics for building the epitaxial structure. Thehydrogen-rich environment in the entire growth process makes it easy toincorporate hydrogen into the material during epitaxial growth. Excesshydrogen can then be driven out of the system by annealing the materialat about 500 deg. C. or above during device fabrication.

In the case of ion implantation, selective areas of the material can bemasked off by photoresist during device fabrication. The unmaskedsurfaces can then be exposed to standard hydrogen ion implantationaccelerated at, typically, between 50 KeV to 350 KeV depending on thepenetration depth desired. In practice, the exact dosage and energylevels required is determined experimentally using Secondary Ion MassSpectroscopy (SIMS) characterization in conjunction withelectroluminescence measurements. Monte Carlo computer simulationtechniques using software such as the popular SRIM and TRIM packages canalso be employed to model the ion implantation process.

Although the prior description has focused on tuning a VCSEL, otherapplication of hydrogenation in VCSEL fabrication are also valuable. Forexample, hydrogenation may be used to adjust the lateral index profilein a VCSEL structure to improve the electrical and optical confinementas previously described for an edge emitter laser. Hydrogenation canalso be used to stabilize the polarization of the light output of aVCSEL. In particular, hydrogenation may be used to create refractiveindex asymmetries in the VCSEL aperture. These asymmetries can be usedto control the polarization of the output. An example of controllingpolarization by creating asymmetries is discussed in U.S. Pat. No.6,304,588 entitled “Method and Apparatus for Eliminating PolarizationInstability in Laterally-Oxidized VCSELs” by Chua et al which is herebyincorporated by reference in its entirety. In this case, the sametechniques may be used, however instead of lateral oxidation, ahydrogenation process is used is used to create the asymmetries.

Linking Circuitry:

An additional application of Hydrogen induced bandgap and index changes(HIBs) is to create passive optical and integrated optic components onthe same wafer as active elements. Various passive and active elementscan be formed on the same wafer and even in the same epitaxial layer.This is accomplished by selective area hydrogenation (ordehydrogenation) during device/circuit fabrication.

Passive optical elements such as waveguides are often used tointerconnect devices. One method of forming such a waveguide is on asemiconductor wafer. To create a waveguide, vertical and lateral opticalconfinement is needed. The layer structure of the wafer providesvertical optical confinement. Vertical confinement dimensions in suchwaveguides are usually much smaller (typically <1 μm) than the lateraldimensions (few μm to tens of μm). Thus the index changes used tomaintain single mode operation of the waveguide are larger in thevertical than in lateral dimensions. The index profile in verticaldirection is given by the layer structure and can therefore becontrolled precisely by adjusting appropriate growth parameter (layerthickness and composition). The relatively small index changes inlateral dimensions are usually produced by etching a ridge in the uppercladding layer to produce a lateral effective index change. Etching aridge involves very well controlled etching. An alternate approach usenarrower lateral dimensions by etching a deep ridge through thewaveguide core. In order to maintain single mode operation a higherbandgap epitaxial material is usually grown beside the ridge with asubsequent regrowth step. However, the regrowth results in a non-planarmorphology of regrown structures.

Hydrogen induced bandgap and refractive index shifting of an InGaAsNwaveguide layer offers a simple planar processing technique forfabricating passive optical components as well as integrating thepassive optical components with active components. As used herein,passive components are broadly defined to include waveguides and similarelements including, but not limited to beam splitter, coupler and taperstructures. Passive elements do not undergo electrical stimulation togenerate light. Conversely, “active components” are defined as anycomponent that generates light or an electrical signal output from anenergy input such as an optical pump or an electrical current. Exampleactive devices include but are not limited to lasers, opticalamplifiers, LEDs, electro optical intensity or phase modulators andphoto detectors.

Many active elements as well as the passive elements can be fabricatedfrom the same layer and even the same epitaxial layer structure. Usuallythe most complex component (e.g. laser structure) determine the layerstructure. Compromises to allow the fabrication of different activeelements are sometimes necessary. Hydrogenation processes define theareas used for the different components. In general, hydrogenation isavoided within laser, amplifier and photo diode sections. In otherdevices such as modulators, a low hydrogenation dose is used to shiftthe band gap. A medium hydrogenation may be used for most passiveelements such as waveguides and a strong hydrogenation around thevarious components to help confine the optical and electrical signals.

Hydrogenation induced band gap and refractive index shifts can also beused to create a desired lateral or modify a given vertical mode profilewithin a waveguide components. For example, the index shown in curve2304 of FIG. 23 may be used within a beamsplitter to concentrate themodes into two primary lobes prior to splitting. The index shown incurve 2204 of FIG. 22 may be used to direct optical energy to one sideof a waveguide when an asymmetric distribution of light in a waveguideis to be used at the perimeter or “lateral edges” of the active andpassive components, a step or other sharply defined hydrogenationprofile can be used to create an optical confining barrier. Alternately,a more gradual hydrogenation may be used to create a desired indexprofile and therefore to design a desired lateral light distribution.Graded index changes may be used to optimize matches between the opticalmode profiles in the various different components and waveguides.

FIG. 14 shows a front cross sectional view of a passive waveguide. InFIG. 14, the waveguide 1400 is formed on a wafer upon which laserssimilar to the laser structure of FIG. 5 has been formed. Thus in theexample embodiment, laser component layers including cladding layers1404, spacer layers 1408 and active layers 1412 are already depositedThe waveguide of FIG. 14 uses the layers that already exist on the waferto form a waveguide.

In waveguide 1400, spacer layer 1408 and active layer 1412 can togethercollectively serve as a set of passive waveguide “core” layers 1414. Thelayer structure provides vertical optical confinement. Adjustinghydrogen concentrations in lateral regions 1416, 1420 creates a lateralvariation in the refractive index in nitrogen containing layers,specifically the core layers. By laterally confining a portion of corelayer 1414, the actual waveguide core 1415 can be formed. In oneexemplary embodiment, because the waveguide core is fabricated from thecore layer which is the same layer used to form a laser active region,both the laser active region and the waveguide core 1400 haveapproximately the same composition.

In FIG. 14, a half tone or gray scale mask 1424 determines the lateralindex profile. When exposed to hydrogen 1428, mask 1424 is toned tocontrol hydrogen diffusion into the waveguide core layers. As usedherein, “toned” is broadly defined to mean any of many methods tocontrol the flow of hydrogen through a mask region. In one exemplaryembodiment, such toning may be done by making the tone mask thicker inareas in which less hydrogen is desired, and thinner in areas where morehydrogen is needed. An example of such a suitable toning material isphotoresist patterned using grayscale photolithography, a well-knownmicro-machining processing technique. At sufficiently high temperaturehydrogen is able to penetrate most materials. However, in practicerelative penetration is the most important factor. Thus using a materialwith varying thickness as a gray scale mask should work for manymaterials. Examples are SiO₂ which has been used for maskedhydrogenation of QW lasers (see e.g., Jackson et al, APL 51, 1629 (1987)or Polyimide which has been used for hydrogenated-channel FETs (e.g.,see Constant et al, Electron. Lett. 23, 841 (1987).

In another exemplary embodiment “toning” may mean the mask material isrelatively impervious to hydrogen, but the mask may include smallapertures that allow hydrogen to pass through such that areas higherhydrogen concentration are “toned” to allow more hydrogen through byhaving a higher concentration of apertures as originally shown in FIG.12 or by using larger apertures.

The lateral distribution of hydrogen in the core layers around theactual waveguide core determines the lateral index of refractionprofile. The lateral index profile determines the shape of the opticalmode propagating in the core and thus the optical mode propagating inthe waveguide. By controlling the lateral distribution of hydrogen inthe core layer, waveguide 1400 may be made into a single mode waveguide.Single mode waveguides are particularly useful for providing stableconditions (no noise due to light coupling between different modes) foraccepting light from or coupling light into other elements including,but not limited to lasers modulators, glass fiber optic cables and lightdetectors.

In the example embodiment of FIG. 14, mask 1424 gradually and asymmetrically thins with increasing distance from the waveguide core.The thinning mask causes the hydrogen concentration and thus therefractive index to gradually increase with increasing distance from thewaveguide core. Although symmetry may be desirable for a standardwaveguide, other applications may prefer an asymmetrical mask toning andthus an asymmetrical hydrogen distribution in lateral regions 1416 andregion 1420. By making a toning less gradual on one side, a steeperchange in index of refraction may be achieved on the one side. A steeperindex change results in higher modal confinement on the one side. Thehigher modal confinement is useful in certain applications. For example,when an asymmetrical beam splitter is needed, tighter modal confinementon one side pulls more light to the one side prior to splitting. In somecases even very asymmetric light distribution are desired. This can forexample be achieved by having an index gradient all over the core regionand not only an asymmetric profile at the edges. Such a situation issketched in FIG. 22.

As previously described, vertical confinement is usually achieved by thedifferent layer materials. In a laser, the confinement in both thelateral and vertical direction is typically very tight to facilitateelectrical pumping current and optical waveguide overlap. However, afterentering a passive waveguide, electrical and optical overlap is nolonger needed. Thus the waveguide vertical and lateral confinementcharacteristics may be adjusted to enhance optical coupling between thewaveguide and devices coupled to the waveguide. Thus a method ofadjusting the vertical confinement factors will be described.

FIG. 15A shows a side cross sectional view of a semiconductor DFB or DBRlaser 1508 positioned such that a facet 1516 is oriented to direct lightinto waveguide 1504. In one embodiment, FIG. 15A may be a side crosssectional view along the core of the waveguide; whereas FIG. 14 shows across sectional view perpendicular to the waveguide direction of thesame waveguide 1400. A slight trench 1520 is etched between the laser1508 and waveguide 1504. Trench 1520 facilitates electrical confinementand isolation by preventing current from traveling from contact 1524through the passive waveguide 1504, thus helping to confine current tosemiconductor laser 1508.

One method of modifying optical vertical confinement is to adjust therefractive index differential between the core layers 1540 1544 and thewaveguide layers 1532, 1536 in the vertical direction (perpendicular tothe waveguide core axis and in the plane of the drawing). Such adjustingcan be done by hydrogenation of the waveguide core. FIG. 15A showsgradually increasing the hydrogen concentration along the waveguide corein the direction indicated by arrow 1512, the refractive index along theaxis of the waveguide core (along a direction parallel to arrow 1512)also gradually increases. Because the cladding layers of the waveguide,layers 1532, 1536 do not contain nitrogen, the bandgap of the claddinglayer remains approximately constant along the axis of the waveguide.Thus, in the example shown, the gradual increase in hydrogenation alongthe waveguide core results in a decrease in the index differentialbetween core layers 1540, 1544 and waveguide layers 1532, 1536. Adecreasing index differential results in mode spreading as the wavepropagates away from laser 1508. This mode spreading improves thevertical optical far field distribution which in turn improves thetransverse far field pattern. FIG. 15B shows the refractive indexplotted at two points, Point A and Point B of in the structure of FIG.15A. The plot shows the index of refraction profile gradually decreasingwith increasing distance from the semiconductor laser.

Thus, by controlling the hydrogen concentration along the waveguide coreas shown in FIG. 15, the optical modal profile in the vertical directioncan be improved. Likewise, by controlling the hydrogen concentration oneither side of the waveguide core as shown in FIG. 14, the optical modalprofile in the lateral direction can be controlled. The improvement ofthe modal profile in both the lateral direction and the verticaldirection can be used to improve the transverse far field pattern.

The improvement of the transverse far field pattern is particularlyhelpful in applications where light from a laser 1608 is being coupledinto a glass fiber 1604 a top view of which is shown in FIG. 16.Different optical modes in the laser and the fiber complicates opticalcoupling. In particular, the angle at which an optical fiber can receivelight, the acceptance angle, is often small, typically less than 20degrees. By creating an appropriate far field pattern, in particular byusing a section of the previously described semiconductor waveguide1612, the input of the fiber can be better matched to the output of thesemiconductor waveguide.

The top view of the laser and fiber structure of FIG. 16 shows section1616 a heavily hydrogenated region to create the lateral confinement foran optical signal in a waveguide 1612 (in one example, a cross sectionof waveguide 1612 is the cross section of waveguide 1400 shown in FIG.14). The gradual reduction of hydrogenation along the waveguide axis isshown by arrow 1620 which might be considered analogous to arrow 1512 ofFIG. 15.

Traditional fiber optic matching techniques include designingtraditional taper structures and fabrication the tapers. (e.g., asdescribed in Y Shani et al. Applied Physics Letters 55, 2389 (1989) orKasaya K, Mitomi O, NaganumaM, Kondo Y and Noguchi Y 1993 A simplelaterally tapered waveguide for low-loss coupling to single-mode fibresIEEE Photon. Technol. Lett. 3 345 and references therein) Taperfabrication uses complicated etching and regrowth operations. Bygradually adjusting the hydrogen concentration in the waveguide section1612 between the laser 1608 and a fiber, such as glass fiber 1604, manydifferent taper equivalent structures may be formed by a simple planarprocessing step. In particular, hydrogen induced bandgap changes can beused to taper the semiconductor waveguide and thereby transform theoptical mode profile between different components on an opto-electronicintegrated circuit (OEIC) or to enable efficient coupling from the OEICinto glass fibers.

The waveguides of FIGS. 14-15 may be used to interconnect and integratemultiple active and passive devices on a single semiconductor wafer. Forexample, semiconductor lasers with monolithically integrated modulatorsare commercially available for InGaAsP systems. FIG. 17 shows a laser1704 and modulator 1708 section coupled by a hydrogen induced bandgapshifted passive waveguide section. Note that modulator and laser can befabricated from the same epi-structure. Modulator and amplifier sectionscan share the same InGaAsN active region; however the amplifier andmodulator sections require active regions with slightly different bandgap energies in order to reduce the absorption loss in the modulatorsection. By employing the hydrogen induced bandgap shift in InGaAsNmaterial this can be easily done. Two-section laser diodes, wheremodulator and amplifier section share the same longitudinal cavity andconventional laser modulator combinations are demonstrated in othermaterials, (e.g., M. Suzuki, Y. Noda, H. Tanaka, S. Akiba, Y. Kushiro,and H. Isshiki, J. Lightwave Technol. 5, 1277˜1987!. Or T. Tanbun-Ek, Y.K. Chen, J. A. Grenko, E. K. Byrne, J. E. Johnson, R. A. Logan, A. Tate,A. M. Sergent, K. W. Wecht, P. F. Sciortine, and S. N. G. Chu, J. Cryst.Growth 145, 902˜1994 or R. M. Lammert, G. M. Smith, S. Hughes, M. L.Osowski, A. M. Jones, and J. J. Coleman, IEEE Photonics Technol. Lett.8, 797˜1996). In traditional systems, the bandgap shift betweenmodulator and amplifier section is normally achieved by selective areagrowth or with a separate regrowth step, which makes the fabrication ofdevices in these other materials systems quite elaborate andcomplicated. The preceding example of a laser modulator combination hasbeen provided as one example to demonstrate how interconnect andintegrate different active elements (laser with different emissionwavelength, optical, light intensity modulator, phase modulators andphoto detectors) can be integrated and interconnected with variouspassive elements such as passive waveguides, beam splitter, (wavelengthselective) coupler and taper structure. The example should not beinterpreted to limit the types and methods by which these devices may beinterconnected.

Using the techniques shown in FIGS. 14 and 15, the control of verticaland lateral index profiles in semiconductor material using hydrogenationcan be used to fabricate other passive devices such as beamsplitters.FIGS. 18-22 show exemplary embodiments of various circuits that may befabricated using the techniques previously described, particularly thetechniques described in FIGS. 14-15. Thus all the structures of FIGS.18-22 may be fabricated on a single wafer with the interconnects betweenthe lasers, diodes, amplifiers and detectors being fabricated on thesame wafer as the devices. This integration greatly facilitates devicefabrication.

In FIG. 18, two methods are shown of coupling a laser diode to a monitordiode. In a first configuration 1804, a beam splitter 1812 splits thesignal from laser diode 1808. Some of the light is guided to a monitordiode 1816 while the remaining light travels along a hydrogenatedsemiconductor waveguide 1818 to a taper 1820 where it couples to otherdevices or waveguides such as a glass fiber optic cable. In a secondconfiguration, a hydrogenated semiconductor waveguide 1830 carriessignals from a laser diode 1828 to a monitor device 1824. Note thatmonitor diode, laser and passive waveguide and splitter may befabricated from the same epilayer.

FIG. 19 shows coupling a laser diode 1904 to various amplifiers 1908using waveguides and beamsplitters formed by hydrogenating lateralregions on a semiconductor wafer. In the example of FIG. 19, anamplifier is a structure that provides optical gain and in one examplehas a structure similar to a laser structure except that no opticalfeedback is provided. In general, an amplifier is any structure thatreceives an optical input and provides gain to that input.

The structure of FIG. 20 can also be completely formed on a singlewafer. FIG. 20 shows an integrated receiver/transmitter modulecomprising a laser diode, a photodiode, waveguide sections and awavelength selective waveguide coupler. The laser diode 2004 outputslight at a first frequency (in this example, 1.55 micrometers) along asemiconductor waveguide 2008 to a taper for output to external devicessuch as a glass fiber cable. The coupler directs an incoming lightsignal (e.g. 1.3 μm) from the glass fiber to photodiode 2016. In theillustrated embodiment, the coupler consists of two waveguide sections2008 and 20012 fabricated very close to each other so that the opticalmodes are coupled via evanescent waves. Coupling length 2020 is chosensuch that light with a wavelength of 1.3 μm is coupled almost completelywhereas 1.55 μm light passes the coupler almost unaffected.

FIG. 21 shows an exemplary embodiment of a complicated circuit, an OEIC(opto-electronic integrated circuit) for optical routing includingvarious beam splitters, light modulators and amplifier sections that maybe fabricated on a single wafer. The circuit includes arrays ofamplifier/attenuators 2104, coupled by various semiconductor waveguides.The purpose of this device is to direct light from a selected entranceto one defined output. For example, in order to connect input 2 withoutput 8, amplifier/attenuator sections close to input 2 and 8 areswitched to light amplification while maintaining all otheramplifier/attenuator sections in a light attenuation state. Thus theonly transparent pass is between gate 2 and 8. By using such astructure, beam splitter internal losses and waveguide attenuation canbe (over)compensated in the amplifier sections. The illustratedconfiguration is particularly suitable for directing one particularinput gate to one or more output gates in a multiplexer type ofstructure.

Although the preceding description includes numerous details includinghydrogen concentrations, optical circuit configurations, differentmethods to change hydrogen concentration, and different laser designs.For example, devices such as lasers are described although othercomponents such as light emitting diodes and amplifiers may also use thetechniques described herein. Fine details such as the active layercomposition as AlGaAsN or InGaAsN are used as examples to facilitateunderstanding of the device, and to provide example structures. However,these details are not intended nor should they be used to limit theinvention as alternate embodiments are understood to be possible withoutchanging the intended invention. Thus the invention should only belimited by the claims, as originally presented and as they may beamended, encompass variations, alternatives, modifications,improvements, equivalents, and substantial equivalents of theembodiments and teachings disclosed herein, including those that arepresently unforeseen or unappreciated, and that, for example, may arisefrom applicants/patentees and others.

1. A method of tuning the wavelength of light output by a semiconductorlight emitting structure after initial operation of that light emittingstructure, comprising: forming a first light emitting device including asemiconductor active region that includes gallium, arsenide and nitride,the semiconductor active region designed to emit light; forming a firstelectrical contact on a first side of the active region and a secondelectrical contact on a second side of the active region, the electricalcontacts to guide electrical current flow through the active region;passing a first current between said first and second electricalcontacts and thus through the active region to thereby cause the lightemitting structure to emit light at a first wavelength; determining thefirst wavelength of the light emitted by the light emitting structure;determining a desired second wavelength output of the semiconductorstructure; and, changing the amount of hydrogen in the active regiondesigned to emit light to partially change the effects of the nitride inthe semiconductor active region such that when said first current flowsfrom the first electrical contact and is directed through the activeregion to the second electrical contact, the wavelength of light emittedby the semiconductor light emitting structure is changed from said firstwavelength to approximately said second wavelength.
 2. The method ofclaim 1 wherein the semiconductor structure is a laser.
 3. The method ofclaim 2, wherein the semiconductor structure is a vertical cavitysurface emitting laser.
 4. The method of claim 3 wherein the changing ofthe amount of hydrogen is done by placing the vertical cavity surfaceemitting laser in a vacuum chamber at a temperature between 200 and 450degrees centigrade and exposing the laser to hydrogen.
 5. The method ofclaim 3 wherein the wavelength is changed by less than 20 nm.
 6. Themethod of claim 3 wherein the wavelength output is kept between 1.3micrometers and 1.55 micrometers.
 7. The method of claim 1 wherein thefirst active region includes AIGaAsN.
 8. The method of claim 1 whereinthe first active region includes InGaAsN.
 9. A semiconductor structureto output light comprising: a cavity including an active region designedto emit light, the active region includes at least gallium, arsenide andnitride, the active region including a sufficient level of hydrogen suchthat the hydrogen partially negates the nitride in the active regionthereby increasing the bandgap of the active region; a first electricalcontact coupled to a first side of the cavity and a second electricalcontact coupled to a second side of the cavity, the first electricalcontact and the second electrical contact to provide electrical currentto the active region and cause the output of light from the activeregion; and the active region of a type in which the concentration ofhydrogen therein has been changed after an electric current is passedbetween said first and second electrical contacts and light of a firstwavelength is output from said active region thereby, said hydrogenconcentration changed from a first concentration to a secondconcentration, said change in hydrogen concentration causing a change inthe wavelength of light output by the active region from said firstwavelength to a second wavelength.
 10. The semiconductor structure ofclaim 9 further comprising a stack of distributed Bragg reflectingmirrors bounding at least one side of the cavity.
 11. The semiconductorstructure of claim 9 further comprising: a cladding layer between theelectrical contact and the active region, the cladding to guide the wavepropagating in the active region.
 12. The semiconductor laser structureof claim 11 further comprising: a GaAsN spacer between the active regionand the cladding layer.
 13. The semiconductor structure of claim 9wherein the active region includes InGaAsN.
 14. The semiconductorstructure of claim 9 wherein the active region includes AIGaAsN.
 15. Asemiconductor wafer comprising: a layer including at least GaAsN; afirst region of the layer forming an active region of a first lightemitting device, the first active region designed to emit light andhaving a first hydrogen concentration to react with the nitride in thefirst region of the layer to result in a bandgap in the first region; asecond region of the layer forming an active region of a second lightemitting device, the second active region designed to emit light andhaving a second hydrogen concentration, the second hydrogenconcentration different from the first hydrogen concentration, thesecond hydrogen concentration to react with the nitride in the secondregion of the layer to result in a bandgap in the second region that isdifferent than the bandgap in the first second region; electricalcontacts coupled to the first active region and the second active regionsuch that when current flows through the first active region and thesecond active region, a frequency of light output from the first activeregion differs from a frequency of light output from the second activeregion; and at least one of said first and second regions of a type inwhich the concentration of hydrogen therein has been changed after anelectric current is passed between said electrical contacts coupled tosaid at least one of said first and second regions and light of a firstwavelength is output thereby, said hydrogen concentration changed from afirst concentration to a second concentration, said change in hydrogenconcentration causing a change in the wavelength of light output by saidat least one of said first and second regions from said first wavelengthto a second wavelength.
 16. The semiconductor wafer of claim 15 wherein,following a change in hydrogen concentration of said at least one ofsaid first and second regions, the wavelength of light output by thefirst active region is between 1000 and 1700 nm and the wavelength oflight output by the second active region is within 200 nm of thewavelength output by the first active region.
 17. The semiconductorwafer of claim 15 wherein, following a change in hydrogen concentrationof said at least one of said first and second regions, the concentrationof hydrogen in the first active region and the second active region isbetween 1×10¹³ cm⁻³ and 1 ×10²⁰ cm⁻³.
 18. The semiconductor wafer ofclaim 15 wherein the first active region and the second active regionare suitable for use as a first laser in a wavelength divisionmultiplexing optical communication system.
 19. The semiconductor waferof claim 15 wherein the layer is epitaxially grown and is approximatelyuniform across the wafer.
 20. The semiconductor wafer of claim 15wherein the first hydrogen concentration and the second hydrogenconcentration are obtained by masking the wafer and exposing the waferto a hydrogen gas.
 21. The method of claim 1, wherein said step ofchanging the amount of hydrogen in the active region comprises the stepof heating the light emitting structure to a selected temperature for aselected period of time to drive hydrogen out of the active region.